Centrifugal pumps are commonly used to pump mixtures of liquids and solids, such as slurry in mineral processing. Particularly in mining, the solid particles of ore in the slurry are highly abrasive. These particles can become trapped between the rotating impeller and the static volute (pump casing) during use, causing wear and abrasion of both the impeller and the volute. This wear reduces the life of the pump and its hydraulic efficiency and leads to greater down-time for repairs.
Conventional centrifugal slurry pumps provide vanes on the gland side of the impeller which reduce the hydraulic pressure at the impeller shaft in order to assist the gland sealing mechanism where the shaft enters the volute. There is normally a small clearance between the vanes and the static volute of the pump. Vanes are also conventionally provided on the suction side of the impeller to discourage slurry from recirculating back into the low pressure suction zone of the pump from the high pressure discharge chamber.
One of the disadvantages of the slurry pumps described above is that the areas between the vanes on the suction side and the gland side of the impeller provide an opening between the impeller and static volute at the periphery of the impeller. Abrasive solid particles from the slurry can enter these spaces and become trapped between the vanes of the impeller and the static volute, causing wear to both the impeller and the volute.
This problem is more prevalent and critical on the suction side of the impeller, where the high pressure liquid inside the discharge portion of the volute tends to flow (through the clearance between the impeller and the static volute) towards the low pressure zone in the suction portion of the pump. Wear on the suction side of the impeller is particularly undesirable, as it causes an increased amount of slurry to recirculate, resulting in a loss of pump hydraulic performance and efficiency. As there is no flow through the gland, wear on the gland side of the impeller is less significant, but still undesirable.
In an attempt to overcome this problem, the casings of some prior art centrifugal pumps (see FIG. 1) are provided with an angled face (3) adjacent to the intake throat (8) of the pump. The angled face (3) of the pump casing is closely aligned with a similar angled face (4) on the suction side of the impeller (2a). Provided a small enough clearance (c) can be achieved between the two angled faces (3,4), a degree of sealing can be achieved between the impeller (2a) and the casing (1).
However, because the faces (3,4) are inclined at an angle to the axis (X--X) of other than 90.degree., the faces (3,4) must be exactly concentric with respect to each other and the axis (X--X) in order to achieve the desired sealing. Any eccentricity on the part of either the impeller angled face (4) or the casing angled face (3) will impair the seal and allow slurry to recirculate back to the intake (8), causing wear and loss of pump efficiency.
Further, to adjust the size of the clearance (c) between the two faces (3,4), the pump must be shut down and the entire impeller (2a,2b) moved towards or away from the casing (1). This is time consuming and expensive. Also, any wear which may occur will be directly on the impeller (2a) or the casing (1), which are both large and expensive parts to replace.